The distribution of maternal RNA sequences is inhomogeneous in the 16-cell embryo (and probably also in the egg) of the sea urchin. This has been demonstrated by means of hybridization in large RNA excess of RNA from the different blastomere types to single-copy DNA complementary to RNA in the egg. Because the hybridization method employed in these experiments reveals the complexity of so-called "complex class" RNA and not of the "prevalent" or abundant species, the study is being extended to the latter. The method chosen is hybridization (and kinetic analysis thereof) involving cDNA made from total Blastomere (micromeres, meso/ macromeres) RNA with single-copy DNA of various origins. These results will be compared in detail with those obgained by an entirely independent method for the identification of prevalent messengers, i.e., translation in a cell-free system of wheat germ origin.